1. Field of the Invention
The present invention relates to a beam direction control element for controlling the directivity of emitted light, i.e., a louver, a method of manufacturing the same, and a variety of devices which employ such a beam direction control element.
2. Description of the Related Arts
Liquid crystal displays (LCDs) are characterized by slim shape, light weight, and low power consumption, and therefore widely employed in various devices such as a thin type television set, a personal digital assistant (PDA), a notebook type personal computer and the like as their display units. Conventional LCDs largely depend on the viewing angle in their display definition, which results in such problems as an inverted image or an invisible image depending on the angle at which one views the screen. In recent years, however, with advancing developments of a compensation film for overcoming the viewing angle dependence and display schemes such as an in-plane switching (IPS) scheme using a horizontal electric field, a vertical alignment (VA) scheme using vertical alignment, and the like, LCDs have been realized to eliminate the viewing angle dependence at any viewing angle, and have a wide range of viewing angle even comparable to a cathode ray tube (CRT). Such LCDs become increasingly popular.
A personal digital assistant, which excels in portability, can be used in a conference or the like in a situation, where a screen displayed on the personal digital assistant is shared by a plurality of persons. Otherwise, the personal digital assistants are used under a variety of environments even in a situation, where information is entered in a public space such as within a train, a airplane or the like. Accordingly, under the environment where a personal digital assistant is shared, as first described, the personal digital assistant preferably provides a widest possible range of viewing angle of the screen, such that a displayed screen can be simultaneously viewed by a plurality of persons. On the other hand, when a personal digital assistant is used in a public place, as second described, an excessively wide range of viewing angle would allow others to look into the screen, thus failing to keep information secret and protect privacy. Accordingly, in such a use environment, the range of viewing angle is preferably limited to such an extent that the user alone can view the screen.
For responding to such requirements for the range of viewing angle, a micro-louver film restricts beams from spreading after they have been emitted from a light source or a display device. The micro-louver film comprises light absorbing slats arranged on a film surface at equal intervals. The slats have a certain height with respect to a direction perpendicular to the film surface, so that an incident beam substantially parallel with the orientation of the slats, i.e., a beam impinging substantially perpendicular onto the film surface can transmit the micro-louver film, whereas light impinging onto the film surface at a large angle to the orientation of the slats, i.e., light obliquely impinging onto the film surface is absorbed by the slats and cannot therefore transmit the micro-louver film. Methods of manufacturing such micro-louver films are disclosed for example, in JP-A-50-092751, WO92/11549, JP-A-6-305066, JP-A-6-294903, and WO02/41048.
Each of JP-A-50-92751 and WO92/11549 discloses a method of manufacturing a micro-louver film by alternately laminating a transparent film and a thin light-absorptive film, melting and compressing the resulting laminate to form a block of a desired thickness, and slicing the micro-louver film from the block in a direction perpendicular to the lamination plane.
A micro-louver film disclosed in JP-A-6-305066 has a structure as illustrated in FIG. 1, where between transparent base film 1 and protection transparent film 4, ionizing radiation-curing resin 2, ionizing radiation-curing resin 2′, and photo-absorbent material 3 are arranged at equal intervals in an in-plane direction of the films. Here, protection transparent film 4 may be omitted. The following description will be focused on a process of manufacturing such a micro-louver film.
First, the process begins with the provision of a mold which is formed with linear salients and recesses of desired widths at a desired pitch on the bottom. Polytetrafluoroethylene (PTFE) is coated on surfaces of the mold except for the surfaces of vertical walls rising from the recesses. Next, a photo-absorbent composition is coated on the vertical walls by electrodeposition. Then, ionizing radiation-curing resin 2 is filled in the recesses of the mold, and transparent base film 1 is placed on and bonded to ionizing radiation-curing resin 2. Next, ultraviolet (UV) rays are irradiated to cure ionizing radiation-curing resin 2 to form a molding of the ionizing radiation-curing resin with the photo-absorbent material to form a laminate film which is then removed from the mold. Ionizing radiation-curing resin 2′ is again filled and cured in recesses of the laminate film, followed by smoothing the surface of the resulting laminate film. Consequently, a micro-louver film is provided as illustrated in FIG. 1. It should be noted that the laminate film manufactured by a process described above in FIG. 1 is not provided with protection transparent film 4. When protection transparent film 4 is added, ionizing radiation-curing resin 2′ and laminate film are covered with transparent film 4 after ionizing irradiation-curing resin 2′ has been filled, and subsequently, ionizing radiation-curing resin 2′ may be cured.
A micro-louver film described in JP-A-6-294903 has a structure as illustrated in FIG. 2, where between transparent base 75 and protection transparent film 78, light absorbers 76 in a desired pattern and optically transparent resin 77 are alternately arranged in an in-plane direction of the films. A method of manufacturing this micro-louver film will be described below. First, material for forming a light absorber is coated over the entire surface of transparent base 75, and a resist for sandblasting is coated on the material in a predetermined thickness, and exposed and developed for forming the pattern of vertically rising light absorbers 76. In this process, a sandblast mask is formed on the light absorber forming material in accordance with a desired pattern. Then, the resulting laminate film is sandblasted through the mask, followed by peeling off the masking resist using a remover. In this process, light absorbers 76 are formed to vertically rise on transparent base 75. Subsequently, UV-curing acrylic resin is coated to fill in gaps between light absorbers 76, and excessive resin is scraped off. The resulting laminate is then irradiated with UV rays. Subsequently, protection transparent film 78 is bonded to the laminate with an adhesive to protect the surface of the laminate. Consequently, the micro-louver film is manufactured as illustrated in FIG. 2. The micro-louver film described in JP-A-6-294903 employs the sandblasting which is anisotropic processing that can achieve a high aspect ratio, so that the photo-absorbent layer can be patterned in a high aspect ratio, which facilitates the control of a range of angle over which beams can spread, according to JP-A-6-294903.
On the other hand, micro-louver 83, i.e., light control element described in WO02/41048, has a structure as illustrated in FIG. 3, where optically transparent film 80A having light absorption areas 81A is arranged on optically transparent film 80B having light absorption areas 81B. A method of manufacturing this micro-louver will be described below. First, each of optically transparent films 80A, 80B is formed with grooves or columnar recesses by molding, casting and extrusion, and/or direct mechanical processing. Next, a light absorbing material is filled in or coated on the grooves or columnar recesses to form light absorption areas 80A, 80B. Then, two optically transparent films 81A, 81B having light absorption areas 80A, 80B, respectively, are bonded to each other with an optically clear adhesive or the like such that light absorption areas 80A, 80B align to each other, thereby completing micro-louver 83.
However, some problems still remain unsolved in the conventional micro-louver films described above.
A first problem lies in difficulties in providing micro-louver films of large areas at low cost due to a large number of manufacturing processes and complicated processes involved in the manufacturing.
For example, in JP-A-50-092751 and WO92/11549, a transparent film and a thin photo-absorbent film are alternately laminated multiple times to form a block, where difficulties are experienced in forming a large film having a uniform thickness, particularly, the thin photo-absorbent film. Also, as the micro-louver film is increased in area, the block must be formed by laminating a larger number of films, resulting in a higher cost and a longer time required for the manufacturing. In addition, difficulties are also experienced in cutting a largest possible and longest possible thin film, with a uniform thickness, from the block made of the laminated films. Even if such a large and long film can be cut from the block, the film suffers from an optically asperous surface and therefore requires a smoothing process, which may involve heating and pressing, for example, leading to a further increase in cost.
The manufacturing method described in JP-A-6-305066 involves a step of forming light absorbers on side surfaces of recesses in a mold, integrating the light absorbers with the ionizing radiation-curing compound or resin to form a molding, and removing the molding from the mold. Therefore, as the micro-louver film will is larger, more difficulties are experienced in accurately controlling the thickness of the light absorbers. Consequently, the light absorbers vary in thickness, resulting in variations in controlling the spreading of light on the surface of the base film to reduce the yield of the micro-louver film. Further, this manufacturing method involves complicated processes and an inevitably large number of steps because the ionizing radiation-curing compound must be again filled and cured, and then processed to smooth its surface after the laminate film is removed from the mold.
The manufacturing method described in JP-A-6-294903 requires the steps of coating, exposing, developing, and peeling off the sandblast masking resist, in addition to the sandblasting intended for the light absorber forming material, in order to form the light absorbers in a desired pattern on the transparent base. This results in an increased number of manufacturing processes and expensive products. In addition, since the sandblasting is used in manufacturing the micro-louver film, the transparent base can be roughen on the surface, and an abrasive can remain on the surface, possibly result in a lower yield.
In the micro-louver described in WO02/41048, an optically transparent film undergoes molding, casting and extrusion, and/or direct machining to form therein grooves or columnar recesses. This manufacturing method requires a large side wall angle to the side wall of the formed groove or columnar recess, causing an impediment to a higher aspect ratio of the light absorbing element. Further, from a viewpoint of manufacturing, it is essentially difficult to provide a single film with a structure having a light absorbing element with a high aspect ratio, which can fully exert the function of louver. For this reason, the micro-louver described in WO02/41048 achieves a higher aspect ratio by providing two optically transparent films each having light absorption areas adjacent to each other. However, with the requirements for the steps of forming grooves or recesses in each of two optically transparent films, filling or coating a photo-absorbent material in or on the grooves or recesses, and adhering the two optically transparent films to each other, WO02/41048 suffers from an increased number of manufacturing processes and a higher cost.
A second problem in the micro-louvers according to the related arts lies in limitations in a direction in which spreading light is limited.
In JP-A-50-092751 and WO92/11549, since a transparent film and thin photo-absorbent film are laminated to create a block as described above, micro-louvers manufactured thereby can simply have a linear pattern which includes the alternation of transparent areas 7 and light absorption areas 5 on the surface of the substrate, as illustrated in FIG. 4. In other words, the light absorption areas cannot be formed to have an arbitrary pattern on the surface of the substrate. Consequently, the resulting micro-louver can restrict light from spreading only in direction 6 in which transparent areas 7 alternate with light absorption areas 5. In a method of overcoming a problem of the ability to control spreading light only in one direction, two micro-louver films are laminated such that their light absorption areas are arranged in a lattice pattern, when viewed from above. This method, however, uses at least two micro-louver films, resulting in an increased thickness of the overall film and a higher cost of the micro-louver.
A third problem in the micro-louvers according to the related arts lies in difficulties in simultaneously achieving a high light transmittance and limiting a range of angle over which beams can spread.
For example, in JP-A-6-294903 which can pattern a photo-absorbent layer with a high aspect ratio, a micro-louver film shown in one embodiment includes light absorbers, each of which has a width of 40 μm, and which are arranged at a pitch of 120 μm. From these dimensions, the transmittance is calculated to be 66.67% (=(120 μm−40 μm)/120 μm), excluding contributions due to losses by reflections on the interface and the like. On the other hand, some commercially available micro-louver films exhibit a transmittance of approximately 65% including losses due to reflections on the interface and the like. In other words, the micro-louver film shown in JP-A-6-294903, though having the photo-absorbent layer which can be patterned with a high aspect ratio, simply exhibits even a lower transmittance than those currently available on the market, in consideration of the losses due to reflections on the interface. Also, since this micro-louver film suffers from the roughened surface of the transparent base due to the sandblasting, and difficulties in completely removing the abrasive, the micro-louver film will disperse transmitted light to further reduce the transmittance. The roughened surface of the base and the remaining abrasive are impediments in uniformly filling the photo-curing resin. Further, the sandblasting tends to advance wider on the bottom (i.e., a surface in contact with the base) of the absorbers to result in tapered recesses. This causes a reduction in aperture ratio, i.e., the ratio of the transparent area to the pitch to further reduce the light transmittance.
The light absorber may be reduced in width in order to improve the light transmittance. JP-A-6-294904 describes that a sandblast mask can be formed to have a line width of several micrometers, but does not describe that the light absorber itself can be processed at this pitch. With the current sandblasting techniques, the processed light absorber has a line width which has a lower limit of approximately 20 μm, thus encountering difficulties in improving the transmittance.
Generally, the optically transparent resin may be increased in width for improving the light transmittance. However, as a wider width of the optically transparent resin results in a lower ratio of the height of the light absorber to the width of the optically transparent resin, beams can transmit the louver over a wider range of incident angle, thus degrading the performance of the micro-louver film.
As appreciated from the foregoing, the related arts have still left an unsolved problem of difficulties in simultaneously achieving a high light transmittance and limiting the range of angle over which beams can be incident on the micro-louver film.